when workTreeItems finds a problem with a parameter, don't go on to process it

when workTreeItems finds a problem with a parameter, don't go on to process it

Part of workTreeItems is trying detect a case
where git porcelain refuses to process a file, and where
git ls-files silently outputs nothing. But, it's hard to perfectly
replicate git's behavior, and besides, git's behavior could change.
So it could be that we warn, but then git ls-files does not skip over
it, and so git-annex also processes it after warning about it.

So, if we think we have a problem with a parameter, display the warning,
and skip processing it at all.

Implementing this was complicated by needing to handle the case where
all command-line parameters get filtered out this way. Which is
different than the case where there are none, because we don't want to
operate on all files in this new case..